The present ivvention relates generally to switching power supply circuits and more particularly to biasing circuits for field effect transistors.
Two of the primary concerns in providing electronic circuits to be flown on spacecraft such as communications satellites are weight minimization and power efficiency. Switching power supplies are used for operation of such communication satellites. Because of their ability to operate more efficiently and at higher switching frequencies than bipolar transistors, field effect transistors (FETs) in general and metal oxide semiconductor field effect transistors (MOSFETs) in particular are more desirable as switching elements for spacecraft switching power supplies. However, in a radiation environment like that of space, MOSFETs may fail to remain "off" when a driver circuit voltage is removed. This is because in a radiation environment, the characteristics of the semiconductor material of the FET change as a function of the accumulated total radiation dose. As a result, the threshold voltage or voltage at which the MOSFET begins to turn on or conduct is reduced as the MOSFET is exposed to cumulative amounts of radiation. Prior art spacecraft switching power supplies have used a separate power converter to supply a bias voltage to the gate electrode of the MOSFETs in the main power supply to prevent such a leakage current when the main power supply is in the "off" state. This extra power converter adds extra weight and increases the space requirements for the switching power supplies.